The present invention is directed to aqueous ink compositions. More particularly, the present invention is directed to ink compositions suitable for use in ink jet printing processes. One embodiment of the present invention is directed to a process for preparing an ink composition which comprises admixing water, a colorant, a base, and polyphosphoric acid, wherein the pH of the ink is adjusted by a process selected from the group consisting of (a) adding polyphosphoric acid to an aqueous solution containing the base, and (b) adding the base to an aqueous solution containing polyphosphoric acid. Another embodiment of the present invention is directed to a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and a base; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding polyphosphoric acid thereto. Yet another embodiment of the present invention is directed to a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and polyphosphoric acid; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding a base thereto. The present invention also includes ink compositions prepared by these processes.
Ink compositions for ink jet printing are known. For example, U.S. Pat. No. 5,062,892 discloses ink jet ink compositions containing oxo-anions (phosphates, polyphosphates, phosphate esters, arsenate, molybdate, sulfate, sulfite, and oxalate) in an amount of from 9 milligrams per liter to 14 weight percent. The oxo anion additive is included in the ink to reduce kogation in the thermal ink jet printers.
Although known inks are suitable for their intended purposes, a need remains for improved ink compositions suitable for use in ink jet printing processes. In addition, there is a need for ink compositions which exhibit improved latency in ink jet printers. Further, there is a need for ink compositions which exhibit improved latency and which also exhibit acceptable waterfastness, pH, viscosity, and surface tension and generate high quality images. Additionally, there is a need for ink compositions with good latency and acceptable pH for which a wide range of solvents is available so that the ink composition can be tailored for compatibility with specific colorants, improved waterfastness, adjusted viscosity, adjusted surface tension, or other important ink characteristics. There is also a need for ink compositions with both improved latency and improved apparent dry times and/or penetration rates into substrates.